The present invention relates to a device for testing voltage, particularly without the consumption of a significant amount of power. Testing may involve merely a determination of whether there exists a voltage above a certain value, or it may involve measurement of voltage more precisely.
It is often desirable to test for voltage in order to determine whether an electrical component is live, for example before service work is carried out, to determine whether an electrical component is functioning properly, or to provide some information about an event which gives rise to a characteristic voltage.
Clearly, simple voltmeters, such as a permanent-magnet moving-coil voltmeter, are used for many voltage measurements, particularly for low voltage D.C. Such instruments may be expensive and bulky and not suitable for high voltage A.C. where it is desirable that little power be drawn, or where for safety a capacitive (rather than direct or ohmic) connection to the voltage source is desired.
Liquid crystals are known which respond to voltage and may therefore be used as part of a voltage testing device. In general, such liquid crystals are long chain molecules that, in the presence of an applied electric field, become aligned and therefore exhibit a quasi-crystalline structure. Molecular configuration is thus a function of electrical field; and, since molecular configuration affects the material's optical properties (principally reflectivity), the material is able to give a visual indication of applied electrical field. As voltage is increased from a low value, first the so-called "threshold voltage" is reached where a realignment of the liquid crystal's molecules begins, causing a visible change. Then the "saturation voltage" is reached at which point the realignment is complete.
Since liquid crystals do not emit light, liquid crystal displays (LCDs) require some external source of light if a change in reflectivity etc. is to be viewed. Daylight may be the source of light, or some special illumination may be provided, generally behind the LCD, and powered either from the electrical component to be tested or from a separate source. The provision of some special illumination is preferably avoided, but daylight is often inadequate to allow a change in reflectivity of small LCDs to be viewed, at least at any practical distance. One solution perhaps is to employ a large LCD, or an array of small LCDs, but this too has problems. If a large LCD or an array of LCDs in parallel is used, a large amount of power is required due to a large capacitance. Alternatively, if an array of LCDs in series is used a large voltage is required to cause the desired change in optical properties. Since the threshold voltage is fixed in advance by the job the device has to do, a series array may not be a possible solution.
We have discovered that this problem can be overcome by increasing the initial input voltage in a certain way. A prior art ammeter may be mentioned at this stage since it employs a transformer together with an array of LCDs. U.S. Pat. No. 4,559,496 to Harnden Jr. et al. discloses a low cost hook-on ammeter for consumer use that includes a split core that may clipped around an insulated conductor carrying the current to be measured. This conductor thus acts as a single turn primary winding around a transformer core. Secondary turns around the core activate a LCD, segments of which are connected to individual taps on the secondary winding. As current in the conductor increases, the secondary voltage increases turning on an increasing number of segments of the LCD.
Whilst such a device may be useful for current measurement in accessible conductors, it is unlikely to be suitable for voltage measurement where limited power is to be drawn, or where accessibility is a problem.